1. Field of the Invention
The present invention relates to a display device, and more particularly, to a backlight unit for a display device.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, much effort has been made to study and develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as a substitute for CRTs. Of these flat panel displays, LCD devices have many advantages, such as high resolution, light weight, thin profile, compact size, and low power supply requirements.
In general, an LCD device includes two substrates that are spaced apart and face each other with a liquid crystal material interposed between the two substrates. The two substrates include electrodes that face each other such that a voltage applied between the electrodes induces an electric field across the liquid crystal material. Alignment of the liquid crystal molecules in the liquid crystal material changes in accordance with the intensity of the induced electric field into direction of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field. Contrary to other types of display devices, the LCD devices are non-luminous display devices in that they cannot display images without some light source (e.g., ambient light or a backlight).
FIG. 1 is a cross-sectional view of an LCD device including a backlight unit according to the related art. As shown in FIG. 1, an LCD device 10 includes a liquid crystal panel LP and a backlight unit BL.
The liquid crystal panel LP includes first and second substrates 22 and 5 facing each other, and a liquid crystal layer 14. On the first substrate 22, a thin film transistor T in a switching region S, a pixel electrode 17 in a pixel region P, and a storage capacitor Cst in a storage region C are disposed. Further, gate and data lines 13 and 15 defining the pixel region P are disposed on the first substrate 22. The thin film transistor T includes a gate electrode 32, a semiconductor pattern 34, and source and drain electrodes 36 and 38. The storage capacitor Cst includes a portion of the gate line 13 as a first electrode and a metal pattern 30 as a second electrode. A gate insulator GI covers the gate line 13 and the gate electrode 32. On the second substrate 5, a plurality of color filters 7a, 7b and 7c corresponding to the respective pixel regions P, a black matrix 6 between adjacent color filters 7a, 7b and 7c, and a common electrode 18 are disposed.
The backlight unit BL is disposed below the liquid crystal panel LP such that a light emitted from the backlight unit BL is incident toward the liquid crystal panel LP. The emitted light passes through the liquid crystal panel LP and a light transmissivity of the liquid crystal layer 14 changes in accordance with the induced electric field between the pixel and common electrodes 17 and 18, and thus images are displayed.
The backlight unit BL is divided into an edge type backlight unit and a direct type backlight unit. While a light source in the edge type backlight unit is disposed at side portions of the liquid crystal panel LP, a light source in the direct type backlight unit is disposed behind the liquid crystal panel LP.
FIG. 2 is a view of an edge type backlight unit of an LCD device according to the related art. As shown in FIG. 2, an edge type backlight unit includes a lamp 52, a lamp guide 54, a light guide panel 56, a reflector 58, and a plurality of optical sheets 60.
The lamp guide 54 is disposed at one side portion of the lamp 52, and thus protects and guides the lamp 52 radiating light. The light guide panel 56 uniforms the radiated light throughout the surface thereof such that the backlight unit BL supplies uniform light to the liquid crystal panel LP (in FIG. 1). The reflector 58 is disposed below the bottom surface of the light guide panel 56 and, thus, reflects the portion of the light from the light guide panel 56 directed downward back toward the light guide panel 56 and the liquid crystal panel LP.
The optical sheets 60 are disposed on a top surface of the light guide panel 56, and include a diffusion sheet 61, first and second prism sheets 62a and 63b, and a protection sheet 63. The diffusion sheet 61 diffuses the light emitted from the light guide panel 56, thereby making it even more uniform. The first and second prism sheets 62a and 63b have surface patterns, where rising and falling portions alternate. The rising and falling portions of the first prism sheet 62a are perpendicular to those of the second prism sheet 63b. The first and second prism sheets 62a and 63b refract and concentrate the light emitted from the diffusion sheet 61 to raise a luminance thereof. The protection sheet 63 protects the prism sheets 62a and 63b, and removes light interference by the first and second prism sheets 62a and 62b. 
Through the components shown in FIG. 2 and explained above, the backlight unit BL generates the light incident on the liquid crystal panel LP (in FIG. 1). To display images having a high luminance, it is necessary that the backlight unit BL generate a high luminance light. However, in the related art backlight unit, a substantial amount of light irradiated from the lamp 52 is lost.
FIG. 3 is a view illustrating a light transmissivity of a backlight unit according to the related art. As shown in FIG. 3, if first light L1 emitted from a light guide panel 56 has about a 100% luminance, then second light L2 passing through optical sheets 60 to a liquid crystal panel LP has about a 60 to 70% luminance, and third light L3 reflected by the optical sheets 60 has about 20 to 30% luminance. Therefore, the related art backlight unit loses a substantial amount of light from the lamp, and thus a luminance of the displayed images is reduced. Furthermore, to compensate for the loss of a substantial amount of light, high power consumption or an expensive backlight unit to emit a high luminance light is required. These problems are present in all types of display devices with a backlight unit.